1. Field of the Invention
This invention relates a waveguide type variable optical attenuator for optical communications, and particularly to a waveguide type variable optical attenuator that has low consumption power and low polarization dependency.
2. Description of the Related Art
The waveguide type variable optical attenuator for optical communications functions such that light with an intensity Pin is attenuated to light with an arbitrary intensity Pout (Pin>Pout).
FIG. 1 shows a conventional waveguide type variable optical attenuator that its variable optical attenuator is composed using waveguides. On a substrate 8 of silica, there are provided couplers 3, 4 and two channel waveguides 5, 6 connecting the couplers 3 and 4, which compose a symmetrical Mach-Zehnder optical circuit. Also, On one channel waveguide 6, there is provided a heater 7 that is connected with a power source 7a. Such a composition is, for example, disclosed in Japanese patent application laid-open No.11-249089 and U.S. Pat. No. 5,956,437.
The coupler 3 branches light being propagated through a single-mode fiber 1, so that one is outputted to the channel waveguide 6 and the other is outputted to the channel waveguide 5. The coupler 4 couples the lights being propagated through the channel waveguides 5, 6 and then outputs it to a single-mode fiber 2. The power source 7a can variably control the electric power supplied to the heater 7. The heater 7 generates heat that increases according to the power supplied from the power source 7a, and the channel waveguide 6 is heated by that heat.
However, in the conventional waveguide type variable optical attenuator, there is a problem that the birefringence of the channel waveguide 6 increases by the heating of the heater 7 and the PDL (polarization dependent loss) increases as the attenuation ratio increases.
FIG. 2A shows the relationship between the heater power and the attenuation ratio of the waveguide type variable optical attenuator in FIG. 1, and FIG. 2B shows the relationship between the heater power and the PDL of the waveguide type variable optical attenuator in FIG. 1. As shown in FIG. 2A, according as the attenuation ratio increases due to an increase in the heater power, the difference (PDL) of attenuation ratio between, of light components to be propagated through the channel waveguide 6, polarized wave (vertical polarized wave) vertical to the substrate 8 and polarized wave (horizontal polarized wave) horizontal thereto becomes greater. Namely, as shown in FIG. 2B, the PDL increases according as the heater power increases.
Accordingly, due to this polarization dependency, it is difficult to obtain an attenuation ratio more than 30 dB.
FIG. 3 shows a waveguide type variable optical attenuator that the two waveguide type variable optical attenuators shown in FIG. 1 are connected optically in tandem. The waveguide type variable optical attenuator thus configured may give an attenuation ratio more than 30 dB. However, in this configuration, there is a problem that the two waveguide type variable optical attenuators cause an increase in consumption power.